Right now I've got a 5-way manifold from KegConnection, and I'd like to add an additional 2-way manifold on the end. I've read that you can do this, but I haven't found any instructions on what I actually need to do.

Is it as simple as taking the little cap off the end of one and screwing the other one into it?

Yes, you just need to get a brass nipple with the same size threads. I can't remember what that is, but you can take the cap to match the size at a place like Home Depot. I did this to connect two 2-way manifolds.